Leon's Blogging

Coding blogging for hackers.

Ruby - Method Missing

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當 ruby 找不到 method 就會 call 這個 method

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class Post
  #自己建立 method_missing 呼叫
  def method_missing(method_name, *args)
    puts "You tried to call #{method_name} with these arguments: #{args}"
    super #default method_missing handling raises a NoMethodError
  end
end
Post.new.submit(1, "Here's a post.")

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class Something
  def method_missing(method, *args, &block)
    puts "You called: #{method}"
    p args
    block.call 7
  end
end

s = Something.new
s.call_method "boo", 5 do |x|
  x.times{ puts "in block" }
end

用找不到 method 會 call method_missing 的特性，直接自己定義 method_missing 去定義呼叫其他的 method

它的執行效率並不好，所以只適合用在沒辦法預先知道方法名稱的情況下

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class Post
  DELEGATED_METHODS = [:username, :avatar]
  def initialize(user)
    @user = user
  end

  def method_missing(method_name, *args)
    if DELEGATED_METHODS.include?(method_name)
      @user.send(method_name, *args)
    else
      super #沒有在設定的 DELEGATED_METHODS 裡面，就呼叫 default method_missing handling raises a NoMethodError
    end
  end
end

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class Post
  def initialize(text)
    @text = text
  end
  def to_s
    @text
  end
  def method_missing(method_name, *args)
    match = method_name.to_s.match(/^hash_(\w+)/) #找前面是 hash_
    if match
      @text << " #" + match[1]
    else
      super
    end
  end
end


post = Post.new("HI")
post.hash_ruby
post.hash_metaprogramming
puts post
#=> HI #ruby #metaprogramming

respond_to?

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post = Post.new
post.respond_to?(:to_s) # => true
post.hash_ruby #再 method_missing 有定義所以呼叫得到
post.respond_to?(:hash_ruby) # => false #但在 respond_to 卻回傳 false

因此必須自己定義 respond_to?

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class Post
  def respond_to?(method_name)
    method_name =~ /^hash_\w+/ || super
  end
end

但是 post.method(:hash_ruby) 還是會出現 NameError: undefined method

所以要改成另一個 respond_to_missing?

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class Post
  def respond_to_missing?(method_name)
    method_name =~ /^hash_\w+/ ||super
  end
end

DEFINE_METHOD REVISITED

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class Post
  def initialize(text)
    @text = text
  end
  def to_s
    @text
  end
  def method_missing(method_name, *args)
    match = method_name.to_s.match(/^hash_(\w+)/)
    if match #有 match 到 hash_ 就建立出新的 method
      self.class.class_eval do
        define_method(method_name) do
          @text << " #" + match[1]
        end
      end
      send(method_name) #並且呼叫 method
    else
      super #沒有就 raises a NoMethodError
    end
  end
end

#當 call post.hash_codeschool 就會定義出下面的 method

def hash_codeschool
  @text << " #" + "codeschool"
end

const_missing

另一個跟 method_missing 一樣的，主要是常數找不到時會 call const_missing

官方文件： method_missing MatchData Regexp

參考文件： 如何設計出漂亮的 Ruby APIs method_missing，一個Ruby 程序員的夢中情人 respond_to? vs. respond_to_missing?

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